Ion beam etching is a process often utilized to prepare samples whose structure is then typically investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This technology is used in particular in research, materials research, and quality control for many materials, such as semiconductors, metals, ceramics, plastics, and the like. To carry out the process, the samples are mounted on a sample stage of an ion beam etching unit, and aligned in the beam path of one or more electron beams. Ion beam etching units are typically high-vacuum units that work with a baseline pressure of 10-6 mbar. The ions most commonly used are argon ions, usually at an acceleration voltage from 1 to 10 kV. The quality of the image resolution in the electron microscope is very substantially dependent, in this context, on the quality of the sample. Among the known ion beam etching processes known in practice are, in particular, ion beam slope etching, ion polishing of SEM samples, the wire shadowing method, and ion beam preparation of standard TEM samples. While the last two methods are used for TEM samples, ion beam slope etching is used to prepare cross-sectional SEM samples. In slope etching, profiles of the sample are exposed using the ion beam, a region of the sample being protected, by a mask arranged on the surface of the sample or aligned with respect to the surface of the sample, from material removal by the ion beam. An ion beam slope etching process that has proven particularly effective for producing high-quality SEM samples is one in which at least two ion beams, preferably three ion beams, are guided onto the sample surface at a predefined angle to one another. This method is disclosed in WO 2008/106815 A2.
In all ion etching methods, and in particular in the case of the method from WO 2008/106815 A2, it is advantageous if the sample is cooled during the ion etching operation. Cooling of the sample allows higher beam power levels, which in turn makes possible (even with very sensitive samples) higher etching rates and consequently high efficiency together with excellent reliability and good sample quality.
Cooling operations are used in a plurality of ion beam etching units, and are utilized for SEM and TEM prepared specimens and for special preparation techniques such as the aforementioned ion beam slope etching method. The samples are usually produced at room temperature, and are consequently stable at room temperature. The principal goal of a cooling operation is therefore to prevent the sample from heating up during ion beam etching, for example in order suppress diffusion processes and structural changes. Sufficient cooling is particularly important for heat-sensitive samples made of organic materials, for example plastics.
In practice, essentially two cooling methods have become established. The first method uses a Peltier element, which is embodied to be small and space-saving but has too little cooling output for many applications. The second cooling method encompasses cooling using a coolant, in particular cooling with liquid nitrogen; it is notable for a high cooling output, but has the disadvantage that safety provisions must be observed with regard to the coolant. Most present-day ion beam etching units operate with the second cooling method, cooling being effected with a cooling apparatus of the kind recited initially. Many of the ion beam etching units on the market, in which the sample must be moved (rotation, oscillation) in order to prevent preparation artifacts, exhibit the problems of insufficient thermal contact between the coolant and sample, and limited temperature measurement of the sample.
A further problem results from the limited coolant supply. In some of the known ion beam etching units, the sample is transferred into the vacuum chamber by means of an air-lock device. For service, or if the sample can no longer be transferred out of the vacuum chamber due to inadvertent detachment from the sample stage, the vacuum chamber must be aerated. For this, the coolant reservoir vessel must be baked out in order to completely vaporize the coolant. For this reason, only a limited supply of coolant is used in the known units. Because moreover no fill level indication or monitoring of the coolant supply is present, the fill level must constantly be monitored and coolant must be replenished as necessary. The coolant supply would not be sufficient for unattended sample preparation over a longer period of time (e.g. overnight). Constant manual replenishment of the coolant necessitates complex safety provisions, such as wearing protective clothing.